1. Field of the Invention
The present invention relates to a method of coupling optical parts, particularly to a method for coupling optical part such as optical waveguides, optical devices, optical fibers, and leads in optical communication and optical interconnection and to a material used for coupling the optical parts, and relates to a method of forming a mirror.
2. Description of the Related Art
An optical circuit is used for various optical information processing systems including optical interconnection and optical communication. In the optical circuit, light outputted from a light source such as an LD and a light-emitting diode is transmitted through a waveguide or an optical fiber and an optical signal of the light is converted further into an electrical signal by means of a photodiode. Thus, in order to manufacture an optical circuit, an optical coupler for coupling the light source with an optical fiber and an optical waveguide and an optical coupler for coupling the optical fiber with an optical fiber or an optical waveguide become necessary.
Further, while further diffusion and high advancement of an optical technique are being expected, drastic technical innovation on packaging of optical devices is also demanded. Namely, module packages that are used easily, manufactured easily and provided inexpensively become necessary. When the development process of electronics is discerned, what is lacking mostly in optical packaging is producibility, and optical device surface packaging (optical surface mounting technology SMT) is proposed for the purpose of meeting such requirements. For example, it is described in document [1] listed below that labor is reduced by mounting optical device packages each containing a lens automatically on a printed board where the optical waveguide is arranged.
[1] Uchida T., Masuda Y. and Akasawa M.: Optical Surface Mount Technology, J.J.A.P., 31, Pt. 1. 5B, pp 1652-1655, May 1991
Further, as to the technique of mounting optical devices in a hybrid manner on the optical waveguide, detailed examination is carried forward with a quartz waveguide on a silicon substrate. This examination is described in document [2] listed below:
[2] Kobayashi M. and Kumiharu K.: Hybrid Optical Integration Technology, The institute of electonic information communications, bulltine, C-I Vol. J77-C-I No.5, pp. 340-351
It is the subject in this application how precisely to fix the chip on a mother board, and attention is given to a self-alignment method using a solder bump that is described in document [3] listed below. Although there are various subjects such that solder is to be reflowned with high positional precision or influence by flux on optical devices, various devices are being made:
[3] Sasaki J., Ito M., Honmou H. and Kanayama Y.: Self-Alignment Packaging of Optical Devices by AuSn Bump Coupling, Shingaku Technical Bulletin, OQE93-145, 1993-12
Further, the multi-chip module (MCM) technique in electronics is applied to the bump packaging on the mother board that is performed in order to simplify modularization of array elements. Research regarding optoelectronic hybrid MCM in which an optical device has been taken into the MCM has also been initiated. This research has been reported in document [4] listed below for instance:
[4] Shimokawa F., Koike S and Matsuura T.: Fabrication of Fluorinated Polyimide Waveguides on Copper-Polyimide Multilayer Substrates for Opto-Electronic Multichip Modules, Proc. of 43rd ECTC, pp. 705-710, May 1993
In order to realize such optical device surface packaging, it is required to emit a waveguide light advancing within a waveguide formed in a plane by some method out of the plane and to make the light from the outside of the plane incident into the waveguide in the plane. To meet such requirements, various methods have been proposed. For example, a processing method of cutting a waveguide obliquely using a dicing saw or the like is described in document [5] listed below as a method of forming a reflecting mirror by working the waveguide end obliquely, and a method of etching obliquely by reactive ions is also described in document [6] listed below.
[5] Uchida et al.: The Institute of Electronic Information Communications in Japan, National Spring Meeting 1993, C-277
[6] Takahara H., Koike S. and Matsui S.: Optical Interconnection Technique of Optoelectronic Hybrid MCM, Shingaku Technical Bulletin, CPM 93-80, 1993-10
Furthermore, a method of utilizing a solder bump at the waveguide end as a reflecting mirror has been proposed in document [7] listed below:
[7] Ito M., Honmo H. and Sasaki J.: PD-SMF Optical Coupling Using Solder Bump, the Institute of Electronic Information Communications in Japan, National Autumn Meeting 1993, C-192
However, since the method of mechanically cutting a waveguide obliquely provides low precision in a processing position, it is not only unsuitable for a minute single-mode type optical circuit, but is also unsuitable for mass production. Further, although the method of etching obliquely with reactive ions can obtain high processing precision since the photolithography technique is applicable, the etching rate therein is comparatively slow. Thus, the etching time gets longer and the throughput is lowered when an object of processing is thick.
Further, in the method of utilizing a solder bump as a reflecting mirror, it is required to place the solder bump with high accuracy at the waveguide end, and moreover it is required to form the solder bump into a desired configuration, either of which cannot be performed easily.
Now, when optical devices such as an optical waveguide and an optical fiber are optically coupled with each other, a method of polishing light incident ends or light emitting ends of optical devices and fixing them thereafter while butting against each other is adopted. This method is referred to as a direct-coupling method.
In order to obtain high optical coupling efficiency in any of the methods described above, however, it is required to have the electromagnetic field distributions in the waveguide and the optical fiber be sufficiently matched. Therefore, it is required to have the thickness of the waveguide layer and optical fiber core diameters coincide with each other and also to reduce axial divergence (deviation) and angle divergence between them at the same time.
To realize this, it is necessary to accurately align the coupling faces between optical parts. However, it is not easy to improve alignment accuracy. Therefore, a method for easily and efficiently optically-coupling optical parts such as optical fibers and optical devices is desired.
FIGS. 1(a) to 1(c) show a general step for optical-coupling two optical fibers.
These optical fibers 1 and 2 have a structure in which cores 1a and 2a are enclosed by cladding 1b and 2b and the cores 1a and 2a have a larger refractive index than the cladding 1b and 2b.
To optically couple the optical fibers 1 and 2, as shown in FIG. 1(a), the optical fiber 2 is secured to a fiber securing portion 3 and the optical fiber 1 is set to a mobile stage 4. Then, an operator moves the mobile stage 4 while observing it with a microscope 5 to accurately align the edge of the core 2a of the optical fiber 2 with that of the core 1a of the optical fiber 1 as shown in FIG. 1(b). After the alignment is completed, the joint between the optical fibers 1 and 2 is welded with an arc discharge apparatus 6.
However, the above method for optically coupling optical parts suffers from a problem in that optical coupling between optical parts cannot easily be performed because it is necessary to previously perform accurate alignment.
A solution for the above problem is therefore desired for optical coupling between optical fibers or a coupler for optically coupling a light-emitting device or light-detecting device with an optical fiber or optical waveguide.
To accurately couple optical parts, Japanese Patent Laid-Open Nos. Sho. 53-108452 and Sho. 64-6909 disclose a method for melting the edges of two optical fibers and coupling them by surface tension. However, these prior art references only disclose the coupling between optical fibers but do not disclose the coupling between other optical parts.
Moreover, accurate optical coupling between optical parts is considered by using couplers.
These couplers are described in the documents listed below:
[8] Optical Communication Device Optics--Light-Emitting and Light-Detecting Devices, Hiroo Yonezu, Kogakutosho, Japan
[9] Optical Fiber Technology In ISDN Age, Katsuhiko Okubo, Rikogakusha, Japan
[10] Optical Communication Handbook, Edited by Hiroshi Hirayama et al., Kagakushinbunsha, Japan
These documents show that an edge emitting laser has a rectangular structure with an active layer of approximately hundreds of nanometers by several microns and its radiation angle ranges between 20 and 60.degree. in the vertical direction and between 5.degree. and 20.degree. in the horizontal direction. A surface emitting LED has a large emitting region diameter of 30 to 40 .mu.m and a radiation angle of approximately 120.degree..
In this connection, a single-mode optical fiber has a core diameter of several to ten microns and a multiple-mode optical fiber has a core diameter of several tens of microns. Therefore, to couple an optical semiconductor device with an optical fiber, accurate alignment on the order of 1 micron is desirable in order to decrease the coupling loss.
When accurately aligning and direct coupling a light-emitting device with an optical fiber by contacting the edge of the device with that of the fiber, more specifically, for direct-coupling of an edge emitting laser with a single-mode optical fiber, the coupling efficiency approachs 30%. For direct-coupling of the edge emitting laser with a multiple-mode optical fiber, the coupling efficiency approachs 50%. For direct-coupling of a surface emitting LED with the multiple-mode optical fiber, the coupling efficiency approachs approximately 6%.
A method for setting a lens between an edge emitting laser and single-mode optical fiber has been proposed as a method for coupling the laser with the fiber. In this case, the coupling efficiency is approximately 50%. However, optical coupling becomes further difficult because the number of parts requiring accurate alignment increases.
For direct-coupling of an optical fiber with a waveguide, a coupling efficiency of 56 to 79% is obtained by equalizing the core diameter of the waveguide edge diameter with that of the optical fiber and preventing misalignment of axes.
However, there is a problem in that the direct-coupling of the waveguide with the optical fiber is not easy because the core diameters of the waveguide and optical fiber are limited and accurate alignment on the order of 1 .mu.m is desired.
Moreover, a method different from the above direct coupling and lens coupling methods is proposed. In Japanese Patent Laid-Open Nos. Sho. 55-43538 and Sho. 60-173508, it is proposed to use a material whose refractive index changes by applying light to the material.
However, the optical coupler connection method proposed in Laid-Open No. Sho. 55-43538 includes a method for manufacturing an optical coupler characterized by applying light to the optical coupler substrate made of a material whose refractive index changes proportionally to a light intensity from a position where light should be inputted or outputted and changing the refractive index of the optical coupler substrate so as to form an optical waveguide in self-alignment. To use the optical coupler, it is necessary to arrange and secure optical parts including optical fibers to be coupled by the optical coupler. Therefore, for example, a hole for inserting an optical fiber is formed on the optical coupler.
In Laid-Open No. Sho. 60-173508, an optical waveguide connection method is proposed which is characterized by setting a phase-change-type photosensitive medium material between two waveguides facing each other, applying light to the photosensitive medium material from both the waveguides, and locally denaturalizing the photosensitive medium so as to form a waveguide for optical coupling.
The reference to set a photosensitive medium material between waveguides to be mutually connected, form a waveguide for optical coupling in it, and thereby decrease a loss due to misalignment of optical axes and it is preferable to set the interval between waveguides to 0.1 mm or less and use ultraviolet rays as the light to be applied to the photosensitive medium material. As a result, although the light spreads due to diffraction in the photosensitive medium material, a waveguide for coupling with a small spread is formed. The optical coupling disclosed in this reference uses connection between waveguides formed in the photosensitive medium and misalignment between waveguides to be connected is also taken over between waveguides in the photosensitive medium. Therefore, it is impossible to correct for a large misalignment exceeding a core diameter.
The following materials can be used for the optical coupler.
For example, Laid-Open No. Sho. 55-43538 mentioned above discloses that a chalcogenide-based amorphous semiconductor or macromolecular material containing photopolymerizable monomer is used for the optical coupler and Laid-Open No.Sho. 60-173508 discloses that the photopolymer made by DU PONT LIMITED, Photoresist KPR (trade name) made by KODAK LOMITED, and U.V. 57 (trade name) made by OPTION CHEMICAL LIMITED are known.
The following are refractive-index imaging materials whose refractive indexes change by applying light to them.
For example, Japanese Patent Laid-Open No. Hei. 2-3081 discloses a material made of thermoplastic polymers, ethylene-based unsaturated monomers, and polymerization initiator. Japanese Patent Laid-Open No. Hei. 2-3082 discloses a material made of interpolymers containing such segments as polyvinyl acetate, polyvinyl butyral, polyvinyl acetal, and polyvinyl formal as the main part, or made of polymerizable binder selected among groups made of the mixture of the segments, ethylene-based unsaturated monomers, and an optical initiator. Japanese Patent Laid-Open No. Hei. 3-50588 discloses a material made of solvent-soluble fluorine-contained polymerizable binder, ethylene-based unsaturated monomers, and photopolymerization initiator. Moreover, Japanese Patent Laid-Open No. Hei. 3-36582 discloses a material made of allyl diglycol carbonate, 2,2,-bis{3,5-dibromo-4-(2-mathasryroiloxiethoxy) phenyl} propane, and a photopolymerization initiator.
However, these materials have a low heat resistance because they use thermoplastic resin and methacryroil-based polymeric products as a binder.